In recent years, an electric power steering system (EPS system) that generates torque electrically is used as a mechanism for assisting a vehicle steering operation. Different from a hydraulic power steering system, the EPS system assists the vehicle steering operation only when a steering operation is performed by a driver. Therefore, the EPS system provides many advantages such as low-fuel consumption.
A motor provided in the EPS system generates torque. For example, a brushless motor, which is driven to rotate by supplying a three-phase AC current, is used in the EPS system. In a case where the brushless motor is used in the EPS system, a DC output supplying a predetermined voltage (for example, 12V) needs to be converted to a phase-shifted AC output in order to supply phase-shifted current to multi-phase (for example, three-phase) coils of the brushless motor. Thus, an electronic control unit is needed to switch over current supply to motor coils. The electronic control unit includes a semiconductor module which performs switching operation.
In a conventional drive apparatus, the electronic control unit is located near an electric motor (for example, disclosed in following patent documents No. 1 to No. 4).
For example, in the patent document No. 1, an electric motor, a heat sink, a power circuit substrate, semiconductor switching elements and a control circuit substrate are stacked in this order. The semiconductor switching elements are mounted on the power circuit substrate in such a manner that a wide surface of each semiconductor switching element is parallel to the power circuit substrate. Therefore, the semiconductor switching element covers a large area on the power circuit substrate, making it difficult to reduce the size of the drive apparatus.